The present disclosure relates to a semiconductor abnormality detection circuit which can be used for detecting occurrence of an abnormality in a semiconductor switch device that on/off-controls energization of a load.
For example, various electric devices are provided as loads of a power source in vehicles. Power from a vehicle-side power source is supplied to such loads via respective switch devices. For example, if a user manipulates a certain button, an electronic control unit (ECU) on the vehicle detects the manipulation and switches the corresponding switch device from an off-state to an on-state. As a result, power from a vehicle-side power source is supplied to a corresponding electric device via the switch device.
In general, switch devices for controlling various electric devices are provided as loads in vehicles are mechanical relays. However, in recent years, they have been being replaced by semiconductor switch devices such as power MOS transistors (FETs). The use of semiconductor switch devices enables reduction in size and weight of devices.
However, semiconductor switch devices may suffer such a failure as a short circuit (between input and output terminals) or an open failure (e.g., disconnection). Furthermore, because of absence of a movable portion such as an electric contact, occurrence/non-occurrence of a failure in a semiconductor switch device cannot be recognized visually or by a working sound.
For example, if a semiconductor switch device is short-circuited, an event may last for a long time that an excessive current flows through the load or the load consumes power wastefully. It is therefore necessary to detect a failure in semiconductor switch devices.
Among conventional techniques relating to detection of a failure of a semiconductor switch device are ones disclosed in JP-A-2008-22152 and JP-A-2010-158108.
JP-A-2008-22152 proposes a technique of a power supply control device which makes it possible to detect, for example, short-circuiting between the input and the output of a semiconductor switch device while suppressing an erroneous detection due to electromotive force of a load motor. More specifically, a judgment signal is generated by comparing a drain-source voltage Vds and a gate-source voltage Vgs of a power MOS transistor (semiconductor switch device) with respective threshold values.
JP-A-2010-158108 proposes a technique of a protective device for a load circuit which enables size reduction of a semiconductor relay and wires. More specifically, a load current is measured by an ammeter and the measured current value is compared with a dead short judging current Imax. Furthermore, a quantity of generated heat and a quantity of radiated heat of a wire are calculated by applying the measured current value to a heat generation equation and a heat radiation equation, respectively.
However, in the technique of JP-A-2008-22152, relatively complicated circuits need to be added to detect voltages Vds and Vgs and compare them with the respective threshold values. Furthermore, since these circuits need to be added for each semiconductor switch, a substantial cost increase is incurred when the number of loads is large.
The technique of JP-A-2010-158108 is associated with various problems relating to the ammeter because it is necessary to directly measure a current flowing from the output terminal of the semiconductor switch to the load. For example, where a detection resistor is inserted in a line through which a current is to flow, the number of wiring steps is increased much and it is necessary to take into consideration adverse effects on the load due to a voltage drop across the resistor. Where a current is detected using a coil that is coupled to a line through a load current is to flow, the ammeter cost may increase to a large extent.
Among methods other than the methods of JP-A-2008-22152 and JP-A-2010-158108 is one in which the voltage across a load is monitored. However, also in this method, many components need to be added and hence a substantial cost increase is incurred when the number of loads is large. Furthermore, occurrence/non-occurrence of a failure cannot be recognized correctly in a situation that a power source voltage is not supplied as in a case that the ignition switch of a vehicle is off.